# a = 'global'

def f1():
    # a = 'f1'
    c = 'c in f1'

    def f2():
        # a = 'f2'
        # c = 'c in f2'

        def f3():
            # a = 'f3'
            # global b
            # b = 101
            # print(a)
            nonlocal c
            print(c)
            c = 'nonlocal variable changed in f3'
            print(c)

        return f3

    return f2

import builtins
builtins.a = 'in builtins'

x = f1()    # x就是f2
y = x()     # y就是f3
y()         # 输出什么？？？
